Secondary batteries are used as a power source for IT electronic devices, such as PDAs, cellular phones, notebook computers, electric bicycles, electric vehicle (EV), hybrid electric vehicle (HEV) and the like. Therefore, there is a growing demand for secondary batteries capable of repeated charges and discharges. In particular, since the performance of these devices depends on the secondary batteries, in particular, high performance secondary batteries are required. Thus the characteristics required for such secondary batteries include excellent charge-discharge characteristics, long life characteristics, high-rate characteristics, thermal stability at high temperatures and the like. In addition, lithium secondary batteries have been drawing attention in terms of use in high voltage and high energy density applications.
Lithium secondary batteries are classified into lithium batteries that use lithium metal as a negative electrode and lithium ion batteries that use carbon negative electrodes that are capable of intercalating/deintercalating lithium ions as an interlayered compound. Furthermore, lithium secondary batteries are often classified by the type of electrolyte used; and they include such as 1 the following: liquid type batteries, gel type polymer batteries and solid state type polymer batteries.
In typical lithium-ion secondary batteries, LiCoO2 is used as the positive electrode material and graphite is used as the negative electrode material. Positive electrode materials that have been researched and developed hitherto include LiNiO2, LiCoxNi1−xO2, LiMn2O4, and other conventional lithium compounds known in the art. LiCoO2 is excellent in terms of stable charge-discharge characteristics and excellent discharge voltage characteristics. However, a cell which uses these materials has disadvantages in that cobalt (Co) is not readily available as a raw material and is thus expensive. In addition, Co has an environmental toxicity factor. Since LiNiO2 is difficult to synthesize and has poor thermal stability, it has not been widely used. Moreover, LiMn2O4 spinel is the most widely used positive electrode material due to its relatively low cost and its ease of synthesis. However, a spinel type of LiMn2O4 electrode for 4V grade secondary batteries has a serious problem in that its theoretical discharge capacity is only about 148 mAh/g, which is much lower in energy density than the other positive Li-ion electrode materials.
Thus, there is a need for a composite oxide positive electrode material having a layered crystal structure and capable of solving various problems including the above recited problems, and at the same time, maintain the advantages of the Co, Ni, Mn oxides. As a measure of potential performance, one equivalent amount of lithium present in a composite oxide having a layered crystal structure can participate in the charge and discharge state, the composite oxide has a theoretical capacity of 285 mAh/g. It is therefore desirable to overcome the above-stated problems and achieve the optimum battery capacity.